Electronic devices typically require a connected (wired) power source to operate, for example, battery power or a wired connection to a direct current (“DC”) or alternating current (“AC”) power source. Similarly, rechargeable battery-powered electronic devices are typically charged using a wired power-supply that connects the electronic device to a DC or AC power source. The limitation of these devices is the need to directly connect the device to a power source using wires.
Wireless power transfer (WPT) involves the use of time-varying magnetic fields to wirelessly transfer power from a source to a device. Faraday's law of magnetic induction provides that if a time-varying current is applied to one coil (e.g., a transmitter coil) a voltage will be induced in a nearby second coil (e.g., a receiver coil). The voltage induced in the receiver coil can then be rectified and filtered to generate a stable DC voltage for powering an electronic device or charging a battery. The receiver coil and associated circuitry for generating a DC voltage can be connected to or included within the electronic device itself such as a smartphone or tablet.
The Wireless Power Consortium (WPC) was established in 2008 to develop the Qi inductive power standard for charging and powering electronic devices. Powermat is another well-known standard for WPT developed by the Power Matters Alliance (PMA). The Qi and Powermat near-field standards operate in the frequency band of 100-400 kHz. The problem with near-field WPT technology is that typically only 5 Watts of power can be transferred over the short distance of 2 to 5 millimeters between a power source and an electronic device, though there are ongoing efforts to increase the power. For example, some concurrently developing standards achieve this by operating at much higher frequencies, such as 6.78 MHz or 13.56 MHz. Though they are called magnetic resonance methods instead of magnetic induction, they are based on the same underlying physics of magnetic induction. There also have been some market consolidation efforts to unite into larger organizations, such as the AirFuel Alliance consisting of PMA and the Rezence standard from the Alliance For Wireless Power (A4WP), but the technical aspects have remained largely unchanged.
Wireless power transfer transmitters commonly have a flat or concave surface on which devices to be charged are placed. Objects other than devices that can be wirelessly charged are sometimes placed on the charging surface of a wireless power transmitter, whether intentionally or inadvertently. Certain metal objects such as coins, paper clips, and keys can develop eddy currents in response to the varying magnetic field produced by the wireless power transmitter. Such “foreign objects” cause losses in the power being transferred to a wirelessly-chargeable device. Also, if left on the surface of the transmitter for a period of time, the eddy currents can cause the foreign metal object to overheat, potentially causing burns or igniting a fire.
The foreign object detection technique set forth in the Qi standard compares the amount of power transmitted by the transmitter and the amount of power that the wireless power receiver reports back to the transmitter. The difference between the two values is a loss number. Eddy currents that develop in a foreign object, such as a coin, on the transmitter surface will cause the loss number to increase. If the loss number exceeds a predetermined threshold, an indicator such as a light emitting diode may be triggered while the transmitter continues to transmit power. If the loss number exceeds the predetermined threshold for a predetermined amount of time, for example ten seconds, the transmitter will stop transmitting power. A drawback of this method of foreign object detection relates to inaccuracy in determining the actual amount of power transmitted or received. For example, a Qi-compliant transmitter indirectly estimates the AC power transmitted by the coil by sensing the DC voltage and current input to the transmitter, which can cause over 300 mW of error at 5 W. In other words, based on the input DC values, when a Qi transmitter determines that it is transmitting 5 W, the actual power may be a value between 4.7 W to 5.3 W. Similarly, a Qi-compliant receiver also indirectly estimates the AC power reaching its coil by sensing the output DC voltage and current, which can cause an error of 300 mW at 5 W. These errors at the transmitter and receiver can compound such that a foreign object escapes detection. For example, a Qi transmitter may be transmitting 5.3 W of power and the Qi receiver may be receiving 4.7 W of power, so the actual power loss is 600 mW, which can be caused by a coin or other foreign object on the transmitter absorbing the 600 mW. But because of errors both the transmitter and receiver sense that the power being transferred is 5 W, so the loss number is determined to be zero and the transmitter keeps operating. Absorbing 600 mW of power can raise the temperature of a coin to a potentially unsafe level. This problem becomes even more potentially dangerous for systems operating at higher power levels such as 10 W. Assuming the same percentage of error, the transmitter and receiver would each have a sensing error of 0.6 W, which could lead to a total power loss of 1.2 W going undetected. A coin on the transmitter absorbing 1.2 W of power going undetected can rise to a temperature that is a definite fire hazard. This inaccuracy in sensing transferred power is a major drawback that prevents Qi-compliant systems from safely delivering more than 5 W. Thus there is a need for an improved technique for detecting foreign objects on wireless power transmitters.